A steam generator is an essential component of reactor plant primary circuit. In its turn, connection of a primary circuit coolant header with a heat-exchange tube bundle is the most complicated steam generator assembly for manufacture. High inter-circuit density requirements are imposed on this assembly. That means that the coolant header connection assembly design shall exclude the possibility of damage of heat-transferring and other components, if their loss of tightness leads to primary circuit radioactive water getting inside the secondary circuit steam-water coolant circulating through the turbine, condensers, heaters and other similar structural components of the NNP reactor plant with potential radioactive substance release to the environment.
The primary circuit coolant header used in horizontal steam generators is generally a thick-walled cylindrical vessel with its diameter and thickness varying along the length of the said vessel. The central cylinder part of headers has through holes for fastening the ends of heat-exchange tubes. The lower cylindrical part of the header is designed with a possibility of weld connection with the steam generator vessel connection pipe and the upper cylindrical part of header is equipped with a hatch (manhole) for access inside and has a conical adapter to the flange connection of the manhole with its lid.
Main problems that can arise during steam generator operation are related to provision of structural integrity of flange connectors and structural integrity of primary circuit coolant headers.
The most labor-consuming and complex operation during steam generator manufacture, from the technological point of view, is the connection of a heat-exchange tube bundle to primary circuit coolant headers involving drilling a large number of closely spaced deep through holes on a limited area of the header side wall followed by leak-tight insertion of heat-exchange tubes in the same. A large number of closely spaced deep through holes reduces the strength of the header, thus, limiting the number of heat-exchange tubes that can be placed in the steam generator with the specified side wall thickness, and if the number of heat-exchange tubes, a significant increase of the header side wall thickness is required.
The above problems are solved differently within the technical background.
Thus, an method to solve the problem of structural integrity of flange connectors is disclosed in USSR Inventor's Certificate No. 1267847, issued on Jan. 10, 1996, IPC: F22B1/02, describing a steam generator assembly comprising a primary circuit heating coolant header with s flange connector located in the steam generator vessel neck forming an annular region. An emergency primary circuit heating coolant flow arrester is installed in the region separating the upper cavity the neck from the rest of the vessel, and it is designed as a seal ring with a cross-section in the shape of a downwards tapered wedge.
Another sealing device used in NPP reactor plant assemblies is disclosed in Russian Utility Model Patent No. 84491, IPC F16J15/12, issued on Jul. 10, 2009. A sealing gasket made of two L-shaped cross section split stop rings with a seal element made of pressed expanded graphite foil in between. Use of expanded graphite foil gaskets in sealing assemblies allows to decrease the probability of reactor plant primary circuit coolant leakage when they are used both for cylindrical and end surfaces of the seal assembly.
Processing technology for deep through holes in steam generator headers, tube sheets and other equipment components of nuclear and petrochemical plants is described in Russian Patent No. 2514359, issued Apr. 27, 2014, IPC B23B35/00. The method includes preliminary drilling of holes with a drilling tool consisting of a head and a stem. At the same time cutting fluid is fed under pressure at least 4 MPa into the gap between the machined surface and the drilling tool and chips are removed by the cutting fluid flow through the inner channels of the head and the stem. Final machining of the surface is performed by reamer installed on the same stem, while chips are removed through the drilled hole forwards in the direction of reaming. Removal of the reamer from the hole is combined with burnishing of the hole surface, while the reamer is rotated at a speed exceeding the rotation speed during reaming up to 4 times and the reamer removal speed exceeds the working feed by 5 to 7%. The use of this method allows to remove a surface layer with residual tension stress, decrease hole surface roughness, increase operation reliability of the heat-exchange equipment connection.
Another invention disclosed Russian Patent No. 2524461, issued on Jul. 27, 2014, IPC B21D39/06, solves the problem heat-exchange pipe end fastening in tube-shaped steam generator headers. According to said invention, tube ends are pre-expanded on the inner surface of the header, tubes are welded, hydraulic expansion within the header thickness is performed, the front end if expanded in the area adjacent to the inner surface followed by mechanical flaring in the area adjacent to the outer surface of the header. The tube front end is expanded by mechanical rolling with 3-roller expanders with torque limitation on the drive shaft. Then one- or two-transition hydraulic expansion is performed. The diameter difference between the mechanical rolling area and the area of hydraulic expansion shall retain up to 0.75÷1% of the heat-exchange tube outer diameter. Application of the invention increases reliability and durability of the connection. The method described is rather labor-consuming and relates only to one operation of heat-exchange tube fastening in the header, and does not cover the full VVER NPP horizontal steam generator primary circuit coolant header manufacture and assembly process.
The closest analog of the proposed technical solution is the one disclosed in Utility Model Patent RU30928 issued on Jul. 10, 2003, IPC: F22B1/02. The utility model relates to manufacture process for a steam generator comprising a vessel and a heat-exchange assembly with inlet and outlet primary circuit coolant headers with a horizontal heat-exchange tube bundle connected to the same. Horizontal heat-exchange tubes are installed at a horizontal and vertical relative spacing of (1.44+−1.55)·d and (1.35+−1.40)·d respectively, where d is the tube diameter. The said spacing range for heat-exchange tube installation is applicable, without limitation, to tight arrangement of heat-exchange bundle tubes, however the issue of heat-exchange tube positioning in a coolant header in case of the said tight arrangement of the heat-exchange bundle remains unsolved. That is, if tight arrangement of heat-exchange tubes is used with horizontal spacing 1.44·d and vertical spacing 1.35·d according to this utility model, coolant header perforated section strength and ease of insertion of heat-exchange tubes into the header during creation of heat-exchange bundle banks are not guaranteed.